Known hinges for composite material parts comprise two metal plates pivoted to each other and provided with holes for the fastening by means of bolts or other threaded member to the parts to be mutually connected. The holes for said known hinges are relatively difficult to be made in the composite material parts and in any case they may cause a structural weakening of these parts, due to the particular physical features of the composite materials. Said known hinges are also relatively difficult to be fastened with adhesives to the composite material parts due to the different physical and chemical features of the materials to be glued.
EP 1738895 discloses a hinge comprising two groups of layers of carbon fibers impregnated with epoxy resin, which layers are arranged on and under a substrate of aramid fibers impregnated with a polyurethane resin. The layers and the substrate are cured together without injection of resins, after which a groove with a V- or U-shaped cross-section is made on both layers, so that a central portion of the substrate is not covered by said layers and forms thus a line of inflection.
However, said known hinge is complex and expensive to be manufactured, since it requires layers of pre-impregnated fibers, known also with the name of pre-preg, which are more expensive than the dry fibers. Furthermore, the outer layers, being more rigid when cured, require a precise mechanic working for making the grooves, which must be perfectly parallel to allow a correct inflection. Said mechanic working, besides increasing manufacture costs and times, also involves the risk of damages of the substrate, since the whole thickness of the rigid layers must be removed, however without leaving the substrate surfaces uncovered. In order to reduce said risk, EP 1738895 explains that the grooves with a V-shaped cross-section are preferable, however these grooves allow only inflections with angles smaller or equal to the summit angle of the groove, thus they cannot be employed in hinges which must rotate with angles greater than 180°. Due to the grooves, the substrate is much thinner than the whole hinge, with the consequent risk of breakings along the line of inflection. On the other hand, the substrate, due to its chemical-physical composition, would be too rigid if it was more or less thick like the rest of the hinge.
The use of different resins for the layers and the substrate further involves adhesion problems which EP 1738895 suggests to decrease through an additional working which roughens the substrate.
Said known hinge is also relatively expensive and delicate due to the aramid and/or woven fibers employed for the substrate. In order to decrease the latter drawbacks EP 1738895 suggests to employ substrates made up of several layers of fibers crossed and/or perpendicular to the line of inflection, with a consequent increase of the manufacture costs. In general, EP 1738895 suggests to employ substrates and layers with fibers similar to each other, wherein the difference of rigidity of the substrate with respect to the layers mainly depends upon the different resins impregnating the layers and the substrate.